This invention relates to a microscopic area scanning apparatus used for scanning over a sample placed on a scanning probe microscope (SPM) that is represented by an atomic force microscope (AFM) or a scanning near optical field microscope (SNOM), wherein at least three hollow cylindrical piezoelectric elements are provided each of which is driven in three XYZ directions by one of divided electrodes.
The scanning probe microscope (SPM) is an apparatus for scanning over a sample surface by using a mechanical prove (hereinafter called "probe") to thereby detect an interaction between the probe and the sample surface, performing observation on the sample surface on the order of nm (10.sup.-6) or lower. For example, the atomic force microscope (AFM) as one typical form of the scanning probe microscope can detect an interatomic force acting between a probe and a sample surface to give information in the form of a change in probe deflection, thus effecting observation on a surface geometry of the sample.
These microscopes require a microscopic area scanning apparatus, in order to scan over samples.
The conventional microscopic area scanning apparatus has three hollow cylindrical piezoelectric elements that are vertically arranged at an interval of an equal angle on a circumference of a common plane. A sample stage is formed by a parallel flat plate, and connected to elastic members having a rotational fulcrum, such as elastic hinges, respectively arranged at free ends of the three hollow cylindrical piezoelectric members. The reason of providing three in number of the hollow cylindrical piezoelectric elements is to support the sample stage at three points. The advantages of the three-point support lies in that the sample stage is placed always in contact with all the free ends of the hollow cylindrical piezoelectric elements regardless of the precision of installation of the hollow cylindrical piezoelectric elements.
Here, the use of the elastic hinges between the hollow cylindrical piezoelectric elements and the sample stage is due to the following reasons.
To scan over the sample stage in a horizontal direction (hereinafter referred to as "XY directions") on a microscopic area scanning apparatus structured as above, the three hollow cylindrical piezoelectric elements have to be deformed by a same displacing amount in a same direction. At this time, the free end of the hollow cylindrical piezoelectric element moves through a circular path about a support end of the hollow cylindrical piezoelectric element. Accordingly, the free end of the hollow cylindrical piezoelectric element increases in inclination as a distance of scan increases.
Considering a case of directly fixing a sample stage on the hollow cylindrical piezoelectric elements, the free ends of the hollow cylindrical piezoelectric elements and the fixing surface of the sample stage to be fixed thereon are kept in contact, in a definite plane, with each other. Where the sample stage is formed by a parallel flat plate with rigidity, the fixing surface of the sample stage cannot follow the tilt in the hollow cylindrical piezoelectric elements. The free ends and the sample stage move away from each other. Due to this, the hollow cylindrical piezoelectric element is acted upon by a bend-resisting force caused by the sample stage. This resisting force reduces the amount of displacing the sample stage with respect to an amount of displacement to be intrinsically caused by the individual hollow cylindrical piezoelectric member. In an extreme case, the hollow cylindrical piezoelectric element cannot withstand to the resisting force, resulting in fracture.
If the sample stage is made by an elastic parallel flat plate in order to avoid such a situation, the fixing surface of the sample stage inclines with increase in inclination of the free ends of the hollow cylindrical piezoelectric element. This, however, causes deflection in the surface of the sample stage. This deflection in turn causes a sample on the sample stage to move following the deflection of the sample stage surface. As a result, when scanning over the sample stage in the XY directions, information on the sample surface geometry detected by the probe positioned above the sample surface is affected by the deflection of the sample stage mixed into the intrinsic sample-surface geometry.
In order to solve the above phenomena, it is usual practice to adopted a method with a structure having a sample stage formed by a rigid member and elastic members having a rotational fulcrum, such as an elastic hinge, arranged for connecting between the sample stage and the hollow cylindrical piezoelectric elements. With this structure, the inclination of the hollow cylindrical piezoelectric element at their free end is absorbed by the deformation of the elastic hinge about its rotational fulcrum. There is no transmission of deflection to a sample on the sample stage, and the information on a sample-surface geometry detected by the probe becomes coincident with the intrinsic sample geometry.
Referring to FIG. 5, there is shown a typical view showing one structural example of a conventional microscopic area scanning apparatus. In FIG. 5, three hollow cylindrical piezoelectric elements 101 (two are shown in the figure) are vertically fixed at an equal interval of 120 degrees on a circumference of a same horizontal plane of the table 105 so that their free ends have coincident heights. Each hollow cylindrical piezoelectric element 101 is axially fixed with an elastic hinge 501 at a top free end. A sample stage 104 is fixed on the totally three elastic hinges 501. The elastic hinge 501 is in a drum form having a neck at a center portion, which is fixed with its drum axis coincident with an axis of the hollow cylindrical piezoelectric element. The neck of the elastic hinge 501 serves as a rotational fulcrum. The elastic hinge 501 is bent depending upon bending deformation of the hollow cylindrical piezoelectric element 101. The material of the elastic hinge 501 uses phosphor bronze that is high in elastic constant and excellent in workability. The material of the sample stage 104 uses an aluminum alloy or stainless steel that has substantial rigidity.
Referring to FIG. 6, there is shown a typical view showing operation of the conventional microscopic area scanning apparatus under scanning in the XY directions, wherein a state is shown that three hollow cylindrical piezoelectric elements 101 (two are shown in the figure) is actuated leftward as viewed on the paper. In this state, the hollow cylindrical piezoelectric elements 101 at their free ends are tilted by a certain angle. On the other hand, the sample stage 104 is kept in a horizontal state. The difference in angle between the free end of the hollow cylindrical piezoelectric element 101 and the sample table 104 is absorbed by bending the neck as a rotational center of the elastic hinge 501.
With such a structure, however, the presence of the elastic hinge decreases a resonant frequency in a height direction (hereinafter referred to as "Z" direction) of the microscopic area scanning apparatus as compared with that of the single hollow cylindrical piezoelectric element. In a scanning probe microscope (SPM), the Z-direction resonant frequency requires at least 1 kHz or higher in order to follow a geometry of a sample surface, even where the scanning rate in the XY directions is at a minimum about 0.5 Hz.
Further, as the scanning range increases broader in the XY directions, the higher the Z-direction resonant frequency is required. In the above-stated structure, the elastic constant of the elastic hinge has to be decreased as the XY scanning range is broadened. Thus, the resonant frequency is lowered. That is, the broad X-Y scanning range and the Z-direction resonant frequency are in a trade-off relationship.